It is well known that kaolin clay can be formed into particles, alone or with other ingredients such as particles of zeolitic molecular sieve, to form coherent bodies such as microspheres which, when calcined, are further hardened. For example, essentially catalytically inert microspheres adapted to be used in a selective vaporization process or to be blended with active zeolite particles are produced by spray drying a slurry of hydrous (uncalcined) kaolin clay arid calcining the resulting microspheres. See U.S. Pat. No. 4,781,818, Reagan, et al. Microspheres consisting of calcined clay and impregnated with precious metal have been commercially used to promote CO combustion in fluid catalytic cracking units. See U.S. Pat. No. 4,171,286, Dight, et al. In some cases, the promoter particles are preblended with particles containing an active cracking catalyst component (usually zeolite Y). In other applications, the promoter particles are introduced at a suitable level into the regenerator of an FCC unit, separately from the particles of cracking catalyst. Still another use of microspheres composed of calcined kaolin clay is as a reactant with caustic or sodium silicate solution to form zeolitic cracking catalyst by so-called in-situ routes. See, for example, U.S. Pat. No. 4,493,902, Brown, et al. Many cracking catalysts are prepared by mixing a slurry of previously formed crystals of zeolite Y in appropriate ion-exchange form with silica sol or silica alumina sol and kaolin followed by spray drying. Spray dried microspheres of calcined clay may also be used as a fluidization additive in FCC units.
In carrying out various processes in which an aqueous slurry of kaolin clay is spray dried, it is conventional to disperse the kaolin in the slurry prior to spray drying in order to permit the formation of high solids slurries that are sufficiently fluid to be spray dried. High solids are preferred for economic reasons. Also, higher solids are conducive to the formation of more strongly bonded particles. To disperse kaolin in water, conventional anionic clay dispersants such as sodium condensed phosphate salts, sodium silicates, soda ash, sodium polyacrylate and mixtures thereof are used. Typically, the pH of concentrated dispersed slurries of kaolin are mildly alkaline to neutral, e.g., 6.0 to 8.0, with pH 7 being optimum.
In many catalytic processes, such as FCC processes, the particles must be attrition-resistant as well as sufficiently porous. Generally, one of these qualities is achieved at the expense of the other. For example, as a particle of given chemical composition is formulated to be highly porous, the hardness usually decreases.
U.S. Pat. No. 5,190,902, Demmel, utilizes the addition of phosphoric acid (or other phosphate compounds) with kaolin clay in a spray drying process to produce spray dried microspheres which are then calcined. In some formulation zeolite particles are present in the spray dryer feed. The process is carried out in one of two basic ways. In one, the slurry of clay particles is brought to a low pH, e.g., 1.0 to 3.0 before being mixed with a source of phosphorus, followed by spray drying. In the other, the clay slurry is brought to a high pH level (e.g., 14.0 to 10.0) before mixing with phosphate-containing compound. According to the teachings of this patent, use of these pH ranges is necessary for the production of particles with superior attrition resistance. A significant problem with these prior art approaches to producing calcined clay microspheres is that neither pH range is the mildly alkaline to neutral pH range at which concentrated slurries of kaolin are fluid and amenable to commercial spray drying using high solids slurries. Thus, patentee diluted his original 70% solids slurry to 40% before pH adjustment apparently because of viscosity increases which follow formation of the aluminum phosphate binder.
Similarly, U.S. Pat. No. 5,231,064, and U.S. Pat. No. 5,348,643, both Absil, et al, describe formation of a cracking catalyst by spray drying a slurry of zeolite with a slurry of clay treated with a phosphorus source at a pH less than 3. Sufficient water is added to bring the combined slurries to a low solids content of ca. 25%.
The use of aluminum phosphates as a binder and hardening agent is well known in the ceramics industry (F. J. Gonzalez and J. W. Halloran Ceram. Bull 59(7), 727 (1980)). This usually involves addition of alumina to the ceramic mix, followed by treatment with phosphoric acid, curing and firing. Similarly, the hardening of aluminous masses such as those composed of bauxite or kaolin clay by incorporation of phosphoric acid followed by heat treatment also is known. The product of this treatment is apparently an aluminum phosphate which can act as a binder. An aluminum phosphate formed by interaction of phosphoric acid solution with an aluminum salt solution has been used to bind zeolite and clay in a cracking catalyst composition (U.S. Pat. No. 5,194,412).